The present invention relates to apparatus and methods for printing and in particular to drop-on-demand (DOD) inkjet printing methods and apparatus.
When DOD inkjet is considered, two main groups can be discerned: thermal inkjet and piezo inkjet.
With thermal inkjet technology, tiny resistors rapidly heat a thin layer of liquid ink. The heated ink causes a vapour bubble to be formed, expelling or ejecting drops of ink through nozzles and placing them precisely on a surface to form text or images. As the bubble collapses, it creates a vacuum that pulls in fresh ink. This process is repeated thousands of times per second. With thermal inkjet technology, water-based inks are used.
Piezoelectric printing technologyxe2x80x94commonly called piezoxe2x80x94pumps ink through nozzles using pressure, like a squirt gun. A piezo crystal used as a very precise pump places ink onto the printing medium. A wide range of ink formulations (solvent, water, UV) may be used.
A number of different piezo concepts exist.
A typical concept, as described in U.S. Pat. No. 4,887,100, WO 96/10488, WO 97/04963 and WO 99/12738, uses so called shared walls. The pressure chambers containing the ink are next to each other, while their dividing walls are the actuators.
Because an actuator is always shared by two channels, it is not possible to jet a drop out of two neighbouring channels at the same time. In WO 96/10488 is described that the nozzles are divided in three interlaced groups (A, B, C). Neighbouring nozzles are fired in a sequence ABC. Two solutions are possible to print dots on a straight line.
A first solution uses a complete nozzle array under a certain angle. By doing this, the resolution is increased, and by using the right fast scan speed, dots fired in a sequence A, B, C are on a straight line.
A second solution uses a head perpendicular to the fast scan direction, in which the A, B, and C nozzles are staggered in the fast scan direction. Printing of a line of pixels is divided into three cycles. In the first cycle, the dividing walls to either side of the A channels are driven (if ink is to be ejected from themxe2x80x94depending on the image to be printed) with a pulsed signal. In the second cycle, the dividing walls to either side of the B channels are driven (if ink is to be ejected from themxe2x80x94depending on the image to be printed) with a pulsed signal. In the third cycle, the dividing walls to either side of the C channels are driven (if ink is to be ejected from themxe2x80x94depending on the image to be printed) with a pulsed signal. The pressure pulses developed in the channels that are not included in the current cycle are not larger than xc2xd of those in the channels that are intended to eject ink. The printing apparatus is arranged so that such pulses with xc2xd magnitude do not cause ink ejection.
A drawback of this concept is that, once the firing frequency is defined, only one fast scan speed can be used to print ABC dots on a straight line, as explained hereinafter. In the fast scan direction, the head will e.g. print each {fraction (1/360)}-inch.
FIG. 1 shows a piezo printhead 10 according to the prior art, having nozzles 12 which are divided into three sets, called a set of A nozzles, a set of B nozzles and a set of C nozzles, each set intended to be fired during different firing cycles. The different sets of nozzles are staggered with respect to each other over a stagger distance D1 in the fast scan direction. If the nozzles are divided in groups G of three, every first nozzle is part of the set of A nozzles, every second nozzle is part of the set of B nozzles and every third nozzle is part of the set of C nozzles. All nozzles in one set A, B, C are positioned on a straight line in the slow scan direction S, which lines are located at the stagger distance D1 with respect to each other in the fast scan direction F.
As an example, printhead 10 is considered to be a type 360 head. This means that the printhead 10 is provided for printing 360 dpi (=pixels per inch) in the fast scan direction F. In this type 360 printhead 10, the distance D1 between nozzles 12 in the fast scan direction F is {fraction (1/360)} inch/3=70.56 xcexcm 3=23.52 xcexcm.
If the firing frequency is 12.4 kHz, meaning that every set A, B, C of nozzles can be fired every 80.65 xcexc, the speed of the printhead 10 in the fast scan direction F is {fraction (1/360)} inch*12.4 kHz=0.875 m/s. The nozzles 12 are fired in an ABC sequence, with the A nozzles at the leading edge of the printhead 10 in the fast scan direction F.
The cycle frequency is 12.4 kHz*3=37.2 kHz. Or formulated in another way: the set of B nozzles fires 26.88 xcexcs after the set of A nozzles, and the set of C nozzles fires 53.76 xcexcs after the set of A nozzles. After 80.65 xcexcs, the set of A nozzles fires again.
One type of printing may be called xe2x80x9cmutually interstitial printingxe2x80x9d, also called shingling e.g. as in U.S. Pat. No. 4,967,203, in which adjacent pixels on a raster line in the fast scan direction are not printed by the same nozzle in the printhead. Printing dictionaries, however, refer to xe2x80x9cshinglingxe2x80x9d as a method to compensate for creep in bookmaking. The inventors are not aware of any industrially accepted term for the printing method wherein no adjacent pixels on a raster line are printed by one and the same nozzle. Therefore, from here on and in what follows, the terms xe2x80x9cmutually interstitial printingxe2x80x9d or xe2x80x9cinterstitial mutually interspersed printingxe2x80x9d are used. It is meant by these terms that an image to be printed is split up in a set of sub-images, each sub-image comprising printed parts and spaces, and wherein at least a part of the spaces in one printed sub-image form a location for the printed parts of another sub-image, and vice versa.
When it would be desired to keep the same firing frequency, but to print a 180*180 dpi image with the 360 type printhead of the example given above, the printhead speed should theoretically double to 1.750 m/s. In the above case of printing a 180*180 dpi image with a 360 type printhead, where the printhead speed must double to 1.750 m/s, the delays for firing B and C need to be shorter to make sure that dots are printed on the same line. Nozzle set B has to be fired 13.44 xcexcs after nozzle set A, and nozzle set C 26.88 xcexcs after nozzle set A. These firing frequencies are too close one to the other, and therefore a 360 type printhead cannot be used to print a 180*180 dpi image.
When it would be desired, on the other hand, to print a 720*720 dpi image with the 360 type printhead, the firing delay between the set of A nozzles, set of B nozzles and set of C nozzles increases to 53.76 xcexc. As, however, after 80.65 xcexcthe set of A nozzles has to fire again, there is not enough time left to fire the set of C nozzles, and therefore a 360 type printhead cannot be used to print a 720*720 dpi image neither.
It is an object of the present invention to provide a method for printing, with one type of printhead, with a resolution which differs from the design resolution of the type of printhead used.
The above objective is accomplished by a method of driving a print head according to the present invention. A print head used has a longitudinal axis in a slow scan direction and has an array of marking elements comprising at least one group of marking elements. Marking elements of one group are staggered with respect to each other over a stagger distance in a fast scan direction, which is perpendicular to the slow scan direction. The print head is intended to be driven with a reference velocity Vref, which is equal to the stagger distance, multiplied by a reference firing frequency Fref. One marking element of a group is able to be fired at each reference firing frequency pulse (whether it fires depends upon the image to be printed). The marking elements of the print head are intended to be fired according to a reference firing order to print an image with a first resolution. The method of the present invention is characterised in that it is operated at an operating velocity that is different from the reference velocity so as to print the same image with a different resolution.
If there are n marking elements in one group, wherein the operating velocity may be equal to       reference    ⁢          xe2x80x83        ⁢    velocity        nX    +    1  
or to             reference      ⁢              xe2x80x83            ⁢      velocity              nX      -      1        ,
X being an integer larger than 0. In the first case. In the first case, the firing order of the marking elements equals the reference firing order, in the second case it equals the inverse of the reference firing order.
The above methods may be used for carrying out fast mutually interstitial printing.
The present invention also includes a printing device with a print head (10) having a longitudinal axis in a first direction (S) and having an array of marking elements (A, B, C; A, B, C, D) comprising at least one group (G) of marking elements (A, B, C; A, B, C, D), marking elements (A, B, C; A, B, C, D) of one group (G) being staggered with respect to each other over a stagger distance (D1) in a second direction (F) perpendicular to the first direction (S), the print head (10) being intended to be driven with a reference velocity (Vref) equal to the stagger distance (D1) multiplied by a reference firing frequency (Fref), one marking element of a group being firable at each reference firing frequency pulse, the marking elements (A, B, C; A, B, C, D) of the print head (10) being intended to be fired according to a reference firing order to print an image at a first resolution, further comprising means for driving the print head (10) at an operating velocity which is different from the reference velocity to print the same image at a second resolution of printing. For this printing device there may be n marking elements (A, B, C; A, B, C, D) in one group (G) and the operating velocity for printing with the second resolution is equal to             reference      ⁢              xe2x80x83            ⁢      velocity              nX      +      1        ,
X being an integer larger than or equal to 0., the firing order of the marking elements (A, B, C; A, B, C, D) to print the second resolution being the same as the reference firing order (ABC; ABCD). Alternatively, this printing device has n marking elements (A, B, C; A, B, C, D) in one group (G), wherein the operating velocity to print the second resolution is equal to             reference      ⁢              xe2x80x83            ⁢      velocity              nX      -      1        ,
X being an integer larger than 0, the firing order of the marking elements (A, B, C; A, B, C, D) to print the second resolution equalling the inverse of the reference firing order (CBA; DCBA).
For either of these arrangements the marking elements (A, B, C; A, B, C, D) of one group (G) may be staggered with respect to each other over a stagger distance (D1) in a second direction (F) perpendicular to the first direction (S) to form a plurality of rows of marking elements, and the printing device may be adapted to supply printing data representing the image to the marking elements of one row which is delayed with respect to the printing data supplied to another row.
The present invention also includes a computer program product for executing any of the methods of the present invention when executed on a computing device associated with a printing head. A machine readable data storage device may store the computer program product. The computer program product may be transmitted over a local or wide area telecommunications network.
The present invention also includes a control unit for a printer for printing an image on a printing medium using a print head (10) having a longitudinal axis in a first direction (S) and having an array of marking elements (A, B, C; A, B, C, D) comprising at least one group (G) of marking elements (A, B, C; A, B, C, D), marking elements (A, B, C; A, B, C, D) of one group (G) being staggered with respect to each other over a stagger distance (D1) in a second direction (F) perpendicular to the first direction (S), the control unit being adapted to control the driving of the print head (10) with a reference velocity (Vref) equal to the stagger distance (D1) multiplied by a reference firing frequency (Fref), and for controlling the firing of one marking element of a group at each reference firing frequency pulse, and for controlling the firing of the marking elements (A, B, C; A, B, C, D) of the print head (10) according to a reference firing order to print the image at a first resolution, further comprising means for controlling the driving of the print head (10) at an operating velocity which is different from the reference velocity to print the image at a second resolution of printing.
Although there has been constant improvement, change and evolution of devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient devices of this nature.